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Semiconductor and metal oxide nanoparticles: Synthesis, physical properties and industrial application

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Shu Li
Abstract:
This thesis addresses various aspects of nanoparticles which attract considerable scientific interest. In chapter 2 and 3, photoionization of individual nanocrystals (CdSe/CdS and CdSe/CdS/ZnS) has been studied by Electric Force Microscopy (EFM). A different response have been observed for two types of high-quality NCs. The magnitude of the charge due to photoionization was found to be wavelength dependent which implies a "hot-electron" process. A new metastable phase of vanadium dioxide hydrate with one-dimensional nanostructure has been synthesized through hydrothermal recrystallization which is described in chapter 4. We propose a hydrating-exfoliating-splitting mechanism to elucidate the formation of nanowires and nanoribbons. Finally, we have conducted synthesis of HfO2 nanoparticles with different crystallinity and morphology. Among them, the amorphous form of HfO2 nanoparticles shows a high solubility in organic solvent which makes it a possible material for high refractive index fluid used in photolithography industry. There is a restriction due to the absorption at the UV region of spectrum. All the details are talked about in chapter 5 and chapter 6.

TABLE OF CONTENTS CHAPTER 1: INTRODUCTION 1 1.1 Nanoscience and Nanotechnology 2 1.2 Semiconductor Nanoparticles 3 1.2.1 Quantum Confinement 4 1.2.2 Fluorescence Blinking 7 1.3 Nanocrystal Synthesis 9 1.3.1 Hydrothermal Synthesis 9 1.3.2 Colloidal Synthesis 12 Referenc 14 CHAPTER 2: ATOMIC AND ELECTROSTATIC FORCE MICROSCOPY 18 2.1 Introduction 19 2.2 Atomic Force Microscopy (AFM) 19 2.3 Electrostatic Force Microscopy (EFM) 21 2.3.1 EFM Theory 21 2.3.2 Individual Tip Calibration 24 2.3.3 Charge Modeling 28 Reference 31 CHAPTER 3: SURFACE STATES IN THE PHOTOIONIZATION OF HIGH-QUALITY CDSE CORE/SHELL NANOCRYSTALS 35 3.1 Introduction 36 3.2 Experimental Section 38 3.2.1 Sample Preparation 38 3.2.2 EFM Set-up 38 3.2.3 Laser Optimization and Others 39 3.3 Results 40 3.3.1 Different Excitation Wavelengths 40 3.3.2 Different Substrates 45 3.3.3 Different Nanocrystals 47 I

3.4 Discussion 48 3.5 Conclusion 54 References 54 CHAPTER 4: SYNETHESIS OF A NEW METASTABLE PHASE OF CRYSTALLIZED V204NH20 NANOWIRES 59 4.1 Introduction 60 4.2 Experimental Section 61 4.2.1 Synthesis 61 4.2.2 Characterization 62 4.3 Results and Discussion 62 4.3.1 VO2 Nanocrystals and Nanoribbons 62 4.3.2 Structure and Composition of Nanomaterials 64 4.3.3 Hydrothermal Recrystallization Mechnism 68 4.3.4 Phase Transition 69 4.4 Conclusion 70 References 71 CHAPTER 5: CRYSTALLINE AND AMORPHOUS HF02 NANOPARTICLES: SYNTHESIS AND STRUCTURAL STUDY 74 5.1 Introduction 75 5.1.1 Hafnia Nanocrystals 75 5.1.2 Nonhydrolytic Sol-Gel Synthesis of HfD2NPs 75 5.1.3 Hydrolitic Synthesis of Amorphous HfC>2 NPs 76 5.1.4 Atomic Pair Distribution Function 77 5.2 Experimental Section 77 5.2.1 Synthesis of Hf02 NCs 77 5.2.2 Synthesis of Amorphous HfC»2 NPs 78 5.2.3 Synthesis of [Hf604(OH)4(OOCMe)12]2 clusters 79 5.2.4 Characterization 79 5.3 Results and Discussion 80 5.3.1TOPO Capped Hf02 NCs 80 5.3.2 Oleate Capped Hf02 NCs 81 5.3.3 Triethylsilanol Capped Hf02 Nanostructures 82 5.3.4 PDF Measurements 83 5.4 Conclusions 89 References 90 ii

CHAPTER 6: TOWARDS THE DESIGN AND DEVELOPMENT OF 193 NM GENERATION THREE IMMERSION FLUID CANDIDATES 93 6.1 Introduction 94 6.1.1 Immersion Fluid Candidates in 193 nm Photolithography 94 6.1.2 Refractive Index Engineering 95 6.1.3 High Refractive Index Material: Hf02 96 6.2 Experimental Section 97 6.2.1 Purification of Chemicals 97 6.2.2 Direct Synthesis of HfC>2 NPs in Immersion Fluids 100 6.2.3 Refractive Index Measurement 104 6.3 Results and Discussion 105 6.3.1 Increase of Refractive Index 105 6.3.2 Problem of Absorption 109 6.4 Conclusions 113 References 114 in

LIST C OF FIGURES AND TABLES CHAPTER 1 Figure 1.1 Size dependent photoluminescent colors of colloidal CdSe NPs dispersed in hexane. Photography by Felice Frankel, from the website of Bawendi group at the MIT Department of Chemistry p.4 Figure 1.2 The continuous conduction and valence energy bands of a bulk semiconductor are separated by a fixed energy gap, Eg, whereas a semiconductor nanocrystal (NC) has discrete atomic-like states and an NC size-dependent energy gap p.5 Figure 1.3 Photoluminescence blinking and on/off time distributions follow power law behavior over a wide range of time scales p.8 Figure 1.4 Scheme of a typical set-up for colloidal synthesis of NPs p.12 Figure 1.5 Scheme of La Mer model for general growth process of colloidal synthesis of NPs p.13 CHAPTER 2 Figure 2.1 Schematic of AFM p.20 Figure 2.2 Schematic of EFM p.22 Figure 2.3 a. Tip geometry used to model the tip-surface capacitance, b. An illustration of the charge distribution in the tip as described by the line and point charge models with relevant geometric parameters p.25 Figure 2.4 a, b. Fitting of the Csphere + Ccone+Cparallel_plale model of tip surface interactions to d2C/dz2 data for two different probes, The dotted lines show experiment data. Solid lines labeled 1-4, respectively, are d2Cl0,/dz2 , d2Csphm/dz2 , d2CconJdz2 , and d2cpa^ei-piate /dz2 contributions to the model p.27 Figure 2.5 Tip geometry and charge distribution p.30 Figure 2.6 Typical plot shows the best fit of dFa (z)/' dz vs z for particles with calculated charge of 1.2 e p.31 CHAPTER 3 Figure 3.1 EFM experimental setup p.39 IV

Figure 3.2 Topography (a) and charge (b-f) images of one area of CdSe/CdS nanocrystals on N-type Si with 14-A SiC>2. (b) charge image before exposure; (c) charge image for green excitation after 60 min; (d) and after 180 min; (e) charge image taken 15 h after the green laser is turned off; (d) charge image taken after UV exposure after 180 min p.40 Figure 3.3 Topography (a) and charge (b-d) images of the same sample area of CdSe/CdS nanocrystals on N-type Si /14-A Si02 with 532-nm light at different intensity for same exposure time: (b) I = 8 mW/cm2 and ton =180 min; (c) I = 15 mW/cm2 and ton =180 min; (d) I = 50 mW/cm2 and ton = 180 min p.41 Figure 3.4 Upper and lower curves in each panel are the calculated signal strengths for a point charge of specified magnitude at the top and bottom of the particle, respectively. The middle curves show the best fit of dFm(z)ldz vs z for particles with calculated charge of 2.4e (a) and 0.8e (b) p.4Error! Bookmark not defined. Figure 3.5 Topographic line scan of several charge profiles for the same CdSe/CdS particle on N-type Si with 14-A SiC>2 and at several different times during photoexcitation experiments: uncharged, before exposure; the observed signals for each particles with 532-nm excitation showing charges of l.le (a), Oe (b), 1.6e (c) and Oe (d); and the observed signals for each particles with 396-nm excitation showing charges of 2.4e (a), 2.2e (b), 1.7e (c). and le (d)p.43 Figure 3.6 Histograms of charge counts observed during the course of photoexcitation experiments on N-type Si with 14-A SiC>2 and with 532-nm and 396-nm excitations p.44 Figure 3.7 UV photoionization study of CdS/CdSe nanocrystals on 300nm thick oxide p.45 Figure 3.8 UV photoionization study of CdS/CdSe nanocrystals on HOPG p.47 Figure 3.9 Topography (a) and charge image (b) of photoionization of CdSe/CdS/ZnS nanocrystals on N-type silicon with 14 A Si02, exposed to 396-nm photoexcitation for 300 min p.48 Figure 3.10 Normalized luminescence excitation spectrum, and optical absorption spectrum of CdSe/CdS nanocrystals in hexane. The luminescence excitation spectrum is a composite of two spectra taken at low and high concentrations p.51 CHAPTER 4 Figure 4.1 Two crystalline structures of VO2 p.61 Figure 4.2 TEM images of different products through hydrothermal recrystallization method: a) V204, b) V205, and c) Ti02 p.62 Figure 4.3 TEM images of products under different conditions: a) raw materials, b) 210 °C for 3 h, c) 210 °C for 12 h, and d) 210 °C for 7 days p.63 Figure 4.4 TEM images of products obtained under different conditions: a) 3 h with pH = 10, b) and c) 12 h with pH = 10, d) 3 h with pH = 2, e) and f) 12 h with pH= 2 p.64 v

Figure 4.5 XRD patterns of the nanowires and nanoribbons p.65 Figure 4.6 XRD patterns of calcined nanowires with different temperature p.66 Figure 4.7 TGA curve for the nanowires and nanoribbons p.67 Figure 4.8 Mechanism of V204- — H20 nanowire and nanoribbon formation from raw V204 8 particles p.68 Figure 4.9 DSC of V02 nanowires and nanoribbons p.70 CHAPTER 5 Scheme 5.1 Hydrolysis and condensation reactions of hafnium tert-butoxide p.77 Figure 5.1 TEM images of TOPO capped Hf02 nanorods and nanoparticles p.80 Figure 5.2 HR-TEM images and XRD patterns of TOPO capped Hf02 nanorods p.81 Figure 5.3 TEM images of oleate capped Hf02 nanorods and nanoparticles p.81 Figure 5.4 HR-TEM images and XRD patterns of oleate capped Hf02 nanorods p.82 Figure 5.5 HR-TEM images and XRD patterns of triethylsilanol capped Hf02 nanostructures p.83 Figure 5.6 300 K PDF (blue circle) compared with PDF refinement (red line). The fitting is as good as Rw = 0.095 p.84 Figure 5.7 PDF of bulk Hf02 in an expanded scale and compared with the relative intensity p.85 Figure 5.8 PDFs of TOPO and Oleic acid capped Hf02 nanoparticles compared with bulk Hf02 p.86 Figure 5.9 PDFs of Hf(OlBu)4 and Hf(OiPr)4 at 300 K compared with bulk Hf02 p.87 Figure 5.10 PDFs of Hf6 clusters dried, in H20 and in hexane p.88 Figure 5.11 PDFs of triethylsilanol capped Hf02 nanostructures in decalin/pentane, precipitate out from organic solvent, and calcined under Ar at 200 °C p.89 CHAPTER 6 Figure 6.1 RI vs. structure from water to perhydropyrene p.95 Figure 6.2 Refractive index of thin Hf02 film p.97 vi

Figure 6.3 Absorbance spectra of decalin (cis / trans mixture) as received from Aldrich, argon purged, and after purification (optical path-length 1 cm) p.98 Figure 6.4 UV spectra of pentane as received (1), after sulfuric acid wash and chromatography (2) and argon purge (3) recorded in 1 cm path-length cell p.99 Figure 6.5 Absorbance spectra of argon purged triethylsilanol before (1) and after distillation (2) p.99 Figure 6.6 UV spectra of distilled isopropanol (1) and argon purged distilled isopropanol (2) p.100 Table 6.1 Exploration of different capping ligands p. 103 Figure 6.7 Sketch of triethylsilanol capped Hf02 nanostructures p.104 Figure 6.8 Refractive index engineering: Hf02 in decalin p. 105 Table 6.2 Refracitve index results with decalin as solvent p.106 Figure 6.9 Refractive index of HfC>2 nanoparticles in DuPont fluid with different concentration p. 108 Figure 6.10 Extrapolation of RI results p.109 Figure 6.11 Absorption of DuPont fluid p.110 Figure 6.12 Absorption of Hf02 nanostructures in DuPont fluid p.l l l Figure 6.13 Absorption of diluted nanoparticle dispersion p.112 Figure 6.14 Absorption spectrum of silanol and silyl ethers p.113 vn

Acknowledgements First and foremost, I would like to thank Prof. Louis Bras for his guidance, encouragements and support over the past five years. From him, I learned how to be a good scientist. I would like to thank to Dr. Michael Steigerwald for all of the many things he has taught me, synthesis, experimental techniques, and all the delighted talk as well. I feel very grateful to have him as my mentor and friend. I also thank my committee members for taking the time to serve on my committee and for their guidance throughout the years, especially Prof. George Flynn for all the valuable advice through five years, and Prof. Nick Turro for providing me with a fantastic opportunity to get exposed to research in industry. I want to thank all my collaborators whom I have had the pleasure of working with. Many thanks to: Dr. Chaya Ben-Porat Rosenthal for being my mentor for EFM operation; Dr. Aran Sundaresan and Dr. Xuegong Lei for helping me with purification of chemicals for Hf02; Prof. Simon Billinge and Peng Tian for their work on Pair Distribution Fuction of Hf02; the current and former members of the Bras group and Flynn group for their help and good company. A most special thank to my family for their constant love and support. I couldn't have done this without them! Vlll

To my family, with love IX

Chapter 1: Introduction Abstract: In this chapter, a brief introduction to nanomaterials will be given. On the base of my graduate study, unique physical properties of semiconductor nanocyrstals and different synthesis route for nanomaterials will be discussed.

2 1.1 Nanoscience and Nanotechnology Nanoscience and nanotechnology have attracted intense attention over several decades, and they are still considered to be the "next industrial revolution". There are broad industrial fields for nanomaterials to be applied into such as biological system and renewable energy regime. The term of "nanotechnology" was first coined by Norio Taniguchi in 1974,1 but as early as 1959, physicist Richard Feynman first sketched the framework of nanoscience and nanotechnology in his famous lecture: "There is Plenty of Room at the Bottom", at the annual American Physical Society meeting. These ideals were realized by Gerd Binnig and Heinrich Rohrer at IBM in 1981.3 They invented scanning tunneling microscope (STM), a powerful tool for viewing surfaces at the atomic level using quantum tunneling effect. And in 1985, Gerd Binnig, Christoph Gerber and Calvin Quate further developed atomic force microscope (AFM) to overcome STM's basic drawback that only conducting surfaces could be investigated. From the material view point, Buckyball (C6o) was discovered by Harold Kroto, Robert Curl and Richard Smalley in the same year of 1985 by using mass spectrometry.4 Nanoscience and nanotechnology were advanced in more vast and far- reaching ways by two of the most fundamental building blocks: one-dimensional carbon nanotubes and zero- dimensional colloidal semiconductor quantum dots which were discovered by Sumio Iijima and Louis Brus, respectively.5' 6 These two works play a key role in the following tremendous improvement in nanoscience and nanotechnology that are made almost daily by scientists and engineers from across the world.

3 My graduate research has focused on three aspects within the big map of nanoscience and nanotechnology: first one is the fundamental physical properties of semiconductor nanocrystals such as photoionization process of CdSe nanocrystals. Second regime is the synthesis of nanomaterials such as metal oxide nanocrystals and amorphous nanoparticles as well. Last aspect is the exploration of applications for nanomaterials into corresponding industry. Typically, I've been working with two different types of nanomaterial: semiconductor nanocrystals and metal oxide nanostructures. 1.2 Semiconductor Nanoparticles When we refer to a nanoscale material, we are speaking of an object that has a size of 1-100 nm in at least one dimension. Thin films can be thought of as two- dimensional nanostructures, nanotubes as one-dimensional nanostructures and nanoparticles as zero-dimensional nanostructures. Among all these nanomaterials, nanoparticles (zero-dimensional nanostructures) have attracted extreme research interest because of their unique properties. They can serve as a bridge between bulk materials and isolated atoms or molecules. A bulk material has constant physical properties regardless of its size which can be successfully described by classical theories, while the properties of a single atom or molecule and the interactions among them are quantum mechanical in nature. NPs consisting of a finite number of atoms possess significant surface to volume ratio and discrete electronic density of states, which lead to many interesting properties such as tunable optical properties of semiconductor NPs as shown in Figure 1.1.

4 Figure 1.1 Size dependent photoluminescent colors of colloidal CdSe NPs dispersed in hexane. Photography by Felice Frankel, from the website of Bawendi group at the MIT Department of Chemistry. 1.2.1 Quantum Confinement The energy band gap of semiconductor materials that separates the conduction band from valence band is important for both fundamental science and practical device applications (Figure 1.2). In bulk materials, the energy band gap is a fixed parameter and only determined by the materials nature. Absorption of a photon by these semiconductor materials promotes an electron from the valence band into the conduction band, which creates an "electron-hole" pair. If the size of semiconductor materials is comparable to or smaller than the natural length scale of electron-hole pair (Bohr radius), it is confined by the boundaries of the materials, which leads to an interesting atomic- (or molecular-) like optical behavior. This phenomenon is known as "quantum size effect", which arises solely due to their finite size. Therefore, semiconductor NPs with the size smaller than Bohr radius are also named as quantum dots (QDs).

5 Bulk semiconductor ^"^ Conduction "^~ —- band —— Energy gap Eg _ ^ _ ^.- ^ „ ^ _ ^ ,— ^ = Valence | s | =z band zg= (bulk) Semiconductor NC 1 1 £9(MC) r Figure 1.2 The continuous conduction and valence energy bands of a bulk semiconductor are separated by a fixed energy gap, Eg, whereas a semiconductor nanocrystal (NC) has discrete atomic-like states and an NC size-dependent energy gap-8 The relation between the electron energy band gap and the size of semiconductor NPs was first developed by Louis Brus by the simple "particle-in- sphere" model9 which considering an arbitrary particle inside a spherical potential well by applying a series of approximations. The "effective mass approximation" and "envelope function approximation"10 completely ignore the semiconductor atoms in the lattice and treats the electron and hole as "free particles". The "strong confinement approximation" justifies the Coulombic attraction between negatively charged electron and positively charged.11 Therefore, the electron-hole pair (ehp) states in a semiconductor NP can be given as

^ehp(^rh)=^e(re)^h{rh) = ucfe(re)uvfh{rh) (1.1) = C u„ JLSKLOYI u. JLh(kr,h,Lhrh)YLh" with energies, (\2)Eehp(nhLhneLe) = Eg h2 2a1 9, »h,Lh K -E„ m eff m eff Where, e is the electron, h is the hole, v means the valence bonds, c means the conduction bonds. 7/"(6,<)>) is a spherical harmonic, je(knJr) is the £th order spherical Bessel function. m°ej means the "effective mass", u is a function with the periodicity of the crystal lattice. The states are labeled by the quantum numbers nhLhneLe. The lowest pair state is written as \Sk\Se. Eg is the semiconductor bandgap and the energies are relative to the top of the valence band. Ec is first order Coulombic attraction energy correction, which is 1.8e /f:afor the electrons in the lSe level, where s is the dielectric constant of the semiconductor material. The selection rules for an optical transition from ground state to a particular electron-hole pair state can also been obtained by this "particle-in-sphere" model. The rules are A«=0 and AL=0. The equation describes that the width of band gap decreases as the NPs size (a2) increases. It is true that the real band structure of semiconductor nanoparticles is more complicated than "particle-in-sphere" model. 13>14-15 And the more accurate band gap structure in semiconductor NPs can be explored by combining theoretical calculations with experimental spectroscopic techniques. The first technique is transient

7 differential absorption (TDA) spectroscopy,16 which is also called pump-probe or hole-burning spectroscopy. It measures the absorption change in semiconductor nanoparticles induced by a spectrally narrow pump beam. The advantage of TDA is its high resolution, while the disadvantage is its strong absorption features overlap with the interest complicate features of the semiconductor nanoparticles. In addition, alterative more convenient technique, photoluminescence excitation (PLE)17 is widely used in today's semiconductor NPs research. The origin of photoluminescence of semiconductor NPs is the recombination of photo activated electron-hole pair (exciton) at the band edge and deactivation of excited electron or hole at the surface states.18 1.2.2 Fluorescence Blinking However, single particle photoluminescence studies showed that fluorescence of individual particles fluctuates with time, as demonstrated in Figure 1.3. That is, the signal blinks on and off on a wide range of time scales. In addition a phenomenon of spectral diffusion was observed in these particles which was correlated with the on/off blinks, suggesting Stark shifts due to charge redistribution in the particle's vicinity.19 It has been postulated that fluorescence "blinking" results from photoionization.19"21

tgif-charged .iparticje^ ... , L iflr—^nltlil uUHaUb 40 Timo (s> 60 80 10' KP 10 Titre (sj 10-- 10-! 10* 10' Timo (S) Figure 1.3 Photoluminescence blinking and on/off time distributions follow power law behavior over a wide range of time scales When a particle has a trapped lone carrier within it, Auger recombination becomes active. Auger recombination is a fast non-emissive process by which the recombination of an electron-hole pair transfers kinetic energy to the third carrier instead of emitting a photon, rendering the particle "dark" until the missing carrier returns. Hence, according to this working hypothesis, the off times correspond the particle being in a charged state. A great number of blinking studies have shown that both on and off times follow the same distribution law in a variety of quantum dot 70 77 ~)(\ system. ' " This universal power law implies the existence of a range of quenching states, coupled to the photoexcited internal state by fluctuating matrix elements even in single passivated core/shell particles made by the best current synthetic methods.21' 77 These processes of fluorescence intermittency and spectral diffusion have been studied for quite a while and are still poorly understood.

9 It also has been found that fluorescence lifetimes of single particles fluctuate by an order of magnitude at room temperature, which is consistent with a model of fluctuating nonradioactive decay channels leading to variability in excited state quenching process28 and also consistent with power law distribution of the fluorescence blinking times. Semiconductor nanoparticles have been deemed as a prime candidate for electro-optical applications due to their size-tunable electronic properties. However, the nanocrystal charge state strongly influences its application in many fields such as photovoltaic devices and photoexcited biological imaging. A lot of studies showed that the behavior of the particles is strongly influenced by fluctuating local electric fields, most likely due to charge redistribution on the particle surface or nearby substrate. 1.3 Nanocrystal Synthesis For nanoscience and nanotechnology, the importance of sample quality can never be overemphasized. Almost all the unique properties and novel applications of nanoscale materials depend dramatically on their size, shape, component, internal structure and derivatization. There are a variety of synthetic methods that have been utilized to make nanocrystals such as hydrothermal synthesis and colloidal synthesis. 1.3.1 Hydrothermal Synthesis Because of its simplicity, low cost, and ambient pressure, crystal growth from solutions has been of considerable interest for a variety of materials and morphologies.

10 As a subset of solvothermal methods, hydrothermal growth has an advantage of being safe and environmentally friendly (growth from aqueous solution instead of organic solvent). In a sealed vessel (bomb, autoclave, etc.), there is an increase of the autogenous pressures resulting from heating, therefore solvents can be brought to temperatures well above their boiling points. Performing a chemical reaction under such conditions is referred to as solvothermal processing or, in the case of water as solvent, hydrothermal processing. The critical point for water lies at 374 °C and 218 atm. Water is supercritical above this temperature and pressure. Supercritical fluids exhibit characteristics of both a liquid and a gas: the interfaces of solids and supercritical fluids lack surface tension, so supercritical fluids exhibit high viscosities and easily dissolve chemical compounds that would otherwise exhibit very low solubilities under ambient conditions.30 Actually, most hydrothermal processes simply take advantage of the increased solubility and reactivity of reactants at elevated temperatures and pressures without bringing the solvent to its critical point. In the past one decade, there is tremendous progress made in the area of hydrothermal technology in the processing of advanced nanomaterials. These materials are believed to have a profound impact on our economy and society in the early part of 21st century. Hydrothermal growth of different nanostructures depends on many apparatus such as growth temperature, precursors, addition of surfactants, solution pH value, crystal properties of the materials and use of substrate as well. Among different

11 types of materials which can be fabricated by this method, metal oxides have been most commonly studied. For metal oxide nanoparticle synthesis, either oxidation of metal sources or hydrolysis (or combination of the two) could be used for synthesis of nanoparticles. One of the most important issues for practical applications is the achievement of precise control of the nanostructure size and orientation. In addition dimensional control of hydrothermally grown oxide nanostructures attract a lot of attention. It has been proposed that the control of the interfacial tension will affect the thermodynamics and kinetics of the nucleation and growth. The surface charge density of metal oxide surface will change the interfacial energy and thus affect the size and distribution of the nanostructures. Actually changing the pH of the solution which results in adsorption of hydroxyl groups or protons will tune the surface charge density. So different pH values in solution will result in different charge densities and electrostatic forces as well, which will in turn affect the aggregation and ripening of the nanostructures. In a later chapter, we will discuss the effect of solution pH value again. Solution composition will also affect the morphology of nanostructures, but it will strongly depend on the crystal properties of the material because the crystal growth rates defer in different directions. Crystal planes which have higher surface energies typically have faster growth rate. However, this can be changed by addition of molecules which selectively adsorb on certain crystal surfaces and it is the mechanism for surfactants. Furthermore, substrate modifications and solution concentrations all have impact on the morphology of final product of nanostructures. It was also found that different materials had different sensitivity to the changes of all

12 these factors, so further work is still required to explore the hydrothermal route for nanomaterials. 1.3.2 Colloidal Synthesis Wet chemical colloidal synthesis is one widely used "bottom-up" approach for I T nanoparticles. The synthesis was significantly advanced by Murray et al. in making II-VI semiconductor nanocrystals using an organometallic route. The advantage of this method is it usually results in high quality nanocrystals with narrow size distribution. Figure 1.4 represents the typical apparatus employed in colloidal synthesis of NPs. The designs can enable flexible control of all necessary parameters for versatile materials by different synthetic routes. And importantly, the NPs growth kinetics can be monitored through the whole process by simply taking out partial reactant from the solution. Figure 1.4 Scheme of a typical set-up for colloidal synthesis of NPs.

13 Mechanism of colloidal NPs growth can be described as the model developed by La Mer and Dinegar. The general process is shown in Figure 1.5. In a typical synthesis, after the precursors and organic surfactants are mixed with the solvent, all the precursors chemically transform into "monomers", which are composed by active atoms or molecules. As soon as the concentration of monomers has achieved the critical concentration (nucleation threshold), monomers could aggregate to form "nuclei". After formation of nuclei, the monomers concentration deceases to a value below the critical concentration, and then the monomers can only add to existing nuclei for continuous growth into NPs. Monomers exchange among particles in order to lower the energy of the whole system, which is referred to a "self-focusing" process and will lead to the narrowest size distribution. When organic surfactants are not sufficient for passivating NPs from aggregation, the "Ostwald ripening" happens, and eventually huge particles precipitate out from the solution. i c <) ** CO 1_ 4 - i c u c o o E o < k. z c Q. \a> \& 19 1 3 A c ctii nje L » — \ • Nucleation Threshold Self- Ostwald X. Focusing Ripening Staturation • Reaction Time

14 Figure 1.5 Scheme of La Mer model for general growth process of colloidal synthesis ofNPs The key point for achieving mono-dispersed NPs is the balance of the chemical reaction speed and the growth speed. Both The concentration of precursors and capping ability of the organic surfactants determine the NPs size. The shape controlled nanoparticles could be acquired by selective adhesion of organic surfactants onto the surface of particles. Once synthesized, the NPs also can be "washed" by polar solvents and precipitated out by centrifuge. Sometime, further treatments, such as thermal annealing and ligands exchange are used for specific applications of NPs. The more detailed procedure of colloidal NPs is described in latter chapters when needed. References: 1. Taniguchi, N. On the Basic Concept of "Nano-Technology".Intl. Conf. Prod. Eng. Tokyo, Part II1974, Japan Society of Precision Engineering: Tokyo, p!8. 2. Feynman, R.P. There's plenty of room at the bottom: An invitation to enter a new field of physics. Caltech's Engineering and Science 1960, 23, 22. 3. Rohrer, G. B. Scanning tunneling microscopy.IBM Journal of Research and Development 1986, 30, 4. Kroto, H. W, ;Heath, J. R.; O'Brien, S. C; Curl, R. F.;Smalley, R. E. C60: Buckminsterfullerene../vaft/re 1985, 318, 6042. 5. Iijimas, S. Helical microtubules of graphitic carbon.Atawre 1991, 453, 6348.

15 6. Rossetti, R.; Nakahara, S.; Brus, L. E. Quantum size effects in the redox potentials, resonance Raman spectra, and electronic spectra of cadmium sulfide crystallites in aqueous solution. J. Chem. Phys. 1983, 79, 1086. 7. Bawendi, M. G. http://nanocluster.mit.edu/. 8. Klimov, V. I. Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties.Marce/ Dekker: New York, 2003, 9. Brus, L. E. Electron-electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. J. Chem. Phys. 1984, 80,4403. 10. Bastard, G. Wave Mechanics Applied to Semiconductor Heterostructures. Wiley: New York, 1991, 11. Efros, A. L. Interband absorption of light in a semiconductor sphere.Sov. Phys. Semiconduct 1982,16, 772. 12. Brus, L. E. A simple model for the ionization potential, electron affinity, and aqueous redox potentials of small semiconductor crystallites. J. Chem. Phys. 1983, 79, 5566. 13. Xia, Jian-Bai Electronic structures of zero-dimensional quantum v/eMs.Physical Review B 1989, 40, 8500. 14. Kittel, Charles, Quantum Theory of Solids. Wiley: New York, 1987. 15. Rosen, Al. L. Efros and M. Quantum size level structure of narrow-gap semiconductor nanocrystals: Effect of band coupling.Physical Review B 1998, 58, 7120. 16. A. P. Alivisatos, A. L. Harris, N. J. Levinos, M. L. Steigerwald, and L. E. Brus Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum.Jott/-«a/ of Chemical Physics 1988, 89, 4001. 17. M. G. Bawendi, W. L. Wilson, L. Rothberg, P. J. Carroll, T. M. Jedju, M. L. Steigerwald, and L. E. Brus Electronic structure and photoexcited-carrier dynamics in nanometer-size CdSe clusters./7!ys7'ca/ Review Letter 1990, 65, 1623. 18. A. A. Guzelian, J. E. B. Katari, A. V. Kadavanich, U. Banin, K. Hamad, E. Juban, and A. P. Alivisatos Synthesis of size-selected, surface-passivated InP nanocrystals. Journal of Chemical Physics 1996,100, 7212.

Full document contains 131 pages
Abstract: This thesis addresses various aspects of nanoparticles which attract considerable scientific interest. In chapter 2 and 3, photoionization of individual nanocrystals (CdSe/CdS and CdSe/CdS/ZnS) has been studied by Electric Force Microscopy (EFM). A different response have been observed for two types of high-quality NCs. The magnitude of the charge due to photoionization was found to be wavelength dependent which implies a "hot-electron" process. A new metastable phase of vanadium dioxide hydrate with one-dimensional nanostructure has been synthesized through hydrothermal recrystallization which is described in chapter 4. We propose a hydrating-exfoliating-splitting mechanism to elucidate the formation of nanowires and nanoribbons. Finally, we have conducted synthesis of HfO2 nanoparticles with different crystallinity and morphology. Among them, the amorphous form of HfO2 nanoparticles shows a high solubility in organic solvent which makes it a possible material for high refractive index fluid used in photolithography industry. There is a restriction due to the absorption at the UV region of spectrum. All the details are talked about in chapter 5 and chapter 6.